1. Field of the Invention
The present invention relates to the field of position sensors and methods, and more particularly to position sensors and methods suitable for sensing and measuring substantial movement.
2. Prior Art
Position sensors and methods are well known in the prior art. Many of these sensors and methods are particularly applicable to instrumentation where the amount of motion or deflection is measured in thousands of an inch or less. There are, however, a number of applications wherein it is desired to measure position of a mechanical element throughout a range of motion of a substantial fraction of an inch or even a number of inches where the environment is less than ideal for a physically sensitive sensor and the mechanical requirements of the assembly cannot be substantially compromised or altered to accommodate the inclusion of the sensor. Such applications include the measurement of lift in internal combustion engine valves wherein the lift is electronically controlled, whether electro-hydraulically, electromagnetically or otherwise. Another application is in free piston engines such as that disclosed in U.S. Patent Application Publication No. US-2011-0083643-A1 entitled “Hydraulic Internal Combustion Engines”. In these applications, position as well as velocity are preferably measured, though velocity may be determined by the time and position difference between two successive position sensings.
U.S. Pat. No. 7,032,549 discloses a valve lift sensor for sensing lift in an internal combustion engine. That sensor uses a conductive target fastened to the engine valve stem that surrounds a coil fixed to the cylinder head. The coil is in series with a resistor with a signal generator energizing the coil with an oscillating voltage to create an oscillating magnetic field concentric to the coil. The oscillating magnetic field induces Eddy currents in the target, which in turn creates reactive magnetic fields. It is stated that “These increase in strength as the valve is opened and the target engages more of the coil magnetic field so that the eddy currents increase and a greater phase shift results between voltage of the signal generator and the current in the coil and the resistor.” “The reactive fields of the Eddy currents affect current flow through the coil by creating a lag in the current phase of the coil 36 and the resistor 38 relative to the phase of the voltage supplied by the signal generator 34”. This phase lag is sensed by a comparator which provides an exclusive OR output signal. “The Exclusive OR output signal is determined according to the following logic. When the state of the signal generator voltage and the resistor voltage are the same, the output signal is “Low”, as illustrated by portions 56 of line 50. However, when the state of the signal generator voltage and the resistor voltage are different, a “High” output signal is generated, as illustrated by portions 52 and 54 of line 50, until the states become the same. Thus, the duty cycle (or width) of the output signals 52, 54 is proportional to the response delay (phase lag) of the resistor voltage, which is proportional to valve lift.”
In the foregoing patent the signal generator voltage is a square wave and the phase lag is sensed at the leading edge and trailing edge of the square wave pulses with a voltage equal to one-half the amplitude of the square wave being taken as the dividing line between the states.
It is not clear how well the above described sensor would operate, or even how it operates. In particular, as the conductive target is lowered around the coil as the engine valve opens, the Eddy currents in the conductive target tend to confine the magnetic field caused by current in the coil, thereby reducing its inductance. At the same time, the Eddy currents increase the apparent resistance of the coil so that the overall effect is to reduce the time constant of the coil, yet it is stated in the patent that the duty cycle or width of the output signals is proportional to the response delay of the resistor voltage which is proportional to valve lift. That duty cycle, as shown in FIG. 3, is a measure of the time constant of the circuit, which would seem to decrease with increasing lift.
In any event, a thin conductive target as shown could be easily damaged during assembly of the engine or by inadvertent contact with hand tools during a repair. Also, while measuring lift on the leading and trailing edge of the square wave excitation has certain advantages, it also has a strong disadvantage. In particular, if the sensing point is not exactly midway between the upper and lower bounds of the square wave excitation, such as by drift of either or both voltages, there will be difference in lift measurement between leading edge measurements and trailing edge measurements. While this difference could be averaged out, two successive readings would need to be taken and then the midpoint between the two readings determined, not easily done and delaying the position determination.